Harnessing the beneficial role of carbon deposit for a superior chemical looping hybrid reforming of methane by flue gas

Suppression of carbon deposition is a widely recognised strategy in high-temperature dry reforming of methane, aimed at ensuring catalyst stability and enabling the simultaneous utilisation of methane and CO 2 . However, in this study, we introduce a novel process - chemical looping hybrid reforming (CLHR) - which takes an opposite approach by intentionally enhancing carbon deposition. This in turn, promotes the mass production of H 2 and CO from methane and CO 2 , respectively. Specifically, an Fe-rich oxygen carrier (OC) derived from fly ash waste was employed to catalyse the cracking of methane in a fuel reactor, resulting in the production of H 2 with a record-high yield of ∼96 mmol/g _OC and a H 2 /CO molar ratio of ∼9.0 at 900 °C. In the subsequent oxidation step, exposure of the reduced OC to a simulated dry flue gas consisting of 16 % CO 2 and 4 % O 2 in N 2 at 900 °C enabled full restoration of its lattice oxygen and catalytic activity, maintaining stable performance over fifty cycles. Consequently, a high-concentration CO stream (∼30 %) with a yield of ∼84 mmol/g _OC was produced. Conversely, the use of flue gas containing steam was found to be detrimental, as it led to the continual growth of carbon nanotubes (CNTs), which progressively encapsulated the reduced OC and ultimately deactivated it. Mechanistically, CNT growth follows the classic vapour-liquid-solid mechanism, in which the presence of metallic Fe 0 and Fe 3 C is essential. Notably, we discovered that steam plays a preferential etching role on the surface of metallic Fe 0 , whereas CO 2 is readily adsorbed on the basic metal oxide interface to remove the amorphous carbon deposited on the CNT walls. These findings provide new insights in harnessing the beneficial role of carbon deposits and advancing the utilisation of methane and CO 2 in a low-emission, efficient, and sustainable manner. • Chemical looping hybrid reforming (CLHR) of methane successfully demonstrated. • Iron rich oxygen carrier with record-high oxygen capacity of 13.4 mmol O/g_ OC. • Dry flue gas (16 % CO 2 + 4 % O 2 ) assumes a stable performance of oxygen carrier. • Effective oxidation of coke by dry flue gas enables high yields of H 2 and CO. • Preferential etching role of steam facilitates CNT growth in wet flue gas.